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Date: Sun, 29 May 2011 00:41:16 +0100
From: MRAB <python@mrabarnett.plus.com>
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Subject: float("nan") in set or as key
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Xref: x330-a1.tempe.blueboxinc.net comp.lang.python:6498

Here's a curiosity. float("nan") can occur multiple times in a set or as 
a key in a dict:

 >>> {float("nan"), float("nan")}
{nan, nan}

except that sometimes it can't:

 >>> nan = float("nan")
 >>> {nan, nan}
{nan}